The Sun Steals Comets from Other Stars   
       
   Nov. 23, 2010:  The next time you thrill at the sight of a comet blazing   
   across the night sky, consider this: it's a stolen pleasure. You're enjoying   
   the spectacle at the expense of a distant star.   
   [...]   
   A cluster of stars forming in the Orion nebula. According to Hal Levison's   
   research, these stars could be swapping comets. [more] Sophisticated computer   
   simulations run by researchers at the Southwest Research Institute (SWRI) have   
   exposed the crime.   
       
   "If the results are right, our Sun snatched comets from neighboring stars'   
   back yards," says SWRI scientist Hal Levison. And he believes this kind of   
   thievery accounts for most of the comets in the Oort Cloud at the edge of our   
   solar system.   
       
   "We know that stars form in clusters. The Sun was born within a huge community   
   of other stars that formed in the same gas cloud. In that birth cluster, the   
   stars were close enough together to pull comets away from each other via   
   gravity. It's like neighborhood children playing in each others' back yards.   
   It's hard to imagine it not happening."   
       
   According to this "thief" model, comets accompanied the nearest star when the   
   birth cluster blew apart. The Sun made off with quite a treasure - the Oort   
   Cloud, which was swarming with comets from all over the "neighborhood."   
       
   The Oort cloud is an immense cloud of comets orbiting the Sun far beyond   
   Pluto. It is named after mid-20th century Dutch astronomer Jan Oort, who first   
   proposed such a cloud to explain the origin of comets sometimes seen falling   
   into the inner solar system. Although no confirmed direct observations of the   
   Oort cloud have been made, most astronomers believe that it is the source of   
   all long-period and Halley-type comets.   
   [...]   
   An artist's concept of the Oort cloud. Note that the distance scale is   
   logarithmic. Compared to the size of planetary orbits, the Oort cloud is very   
   far away. Indeed, the estimated size of the Oort cloud, 10^5 AU, is   
   approximately 1 light year. If the Sun passed within 2 light years of another   
   sun-like star, the stars' Oort clouds would overlap and their comets would   
   intermingle. Image credit: ESO. [more]   
       
   The standard model of comet production asserts that our Sun came by these   
   comets honestly.   
       
   "That model says the comets are dregs of our own solar system's planetary   
   formation and that our planets gravitationally booted them to huge distances,   
   populating the cloud. But we believe this kind of scenario happened in all the   
   solar systems before the birth cluster dispersed."   
       
   Otherwise, says Levison, the numbers just don't add up.   
       
   "The standard model can't produce anywhere near the number of comets we see   
   [falling in from the Oort Cloud]. The Sun's sibling stars had to have   
   contributed some comets to the mix."   
   [...]   
   Could this comet rock-star have been stolen from another stellar system? No   
   one knows. Read more about Comet Hartley 2 here. Comets in the Oort Cloud are   
   typically 1 or 2 miles across, and they're so far away that estimating their   
   numbers is no easy task. But Levison and his team say that, based on   
   observations, that there should be something like 400 billion comets there.   
   The "domestic" model of comet formation can account for a population of only   
   about 6 billion.   
       
   "That's a pretty anemic Oort Cloud, and a huge discrepancy - too huge to be   
   explained by mistakes in the estimates. There's no way we could be that far   
   off, so there has to be something wrong with the model itself."   
       
   He points to the cometary orbits as evidence.   
       
   "These comets are in very odd orbits - highly eccentric long-period orbits   
   that take them far from our Sun, into remote regions of space. So they   
   couldn't have been born in orbit around the Sun. They had to have formed close   
   to other stars and then been hijacked here."   
       
   This means comets can tell us not only about the early history of the Sun -   
   but also about the history of other stars.   
       
   "We can study the orbits of comets and put their chemistry into the context of   
   where and around which star they formed. It's intriguing to think we got some   
   of our 'stuff' from distant stars. We're kin."   
       
       
   Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA   
       
   More Information   
   Oort Cloud -- wikipedia   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.57   
    * Origin: NCS BBS (1:3828/7)